Alloy steel for hot forging and heat treatment method thereof

ABSTRACT

Disclosed is an alloy steel for hot forging and a heat treatment method thereof, more particularly, an alloy steel for hot forging and a heat treatment method thereof, which includes iron as a base material, and about 0.08 to 0.13% by weight of carbon (C), about 1.3 to 1.5% by weight of manganese (Mn), and about 0.1 to 0.3% by weight of molybdenum (Mo), based on a total weight of the alloy steel. As such, a tempering process can be omitted to reduce process costs, while still providing excellent strength and toughness as compared to the alloy steel in the related art.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119 to Korean Patent Application No. 10-2012-0154581, filed on Dec. 27, 2012, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an alloy steel for hot forging and a heat treatment method thereof, and more particularly, to an alloy steel for hot forging which enables omission of a tempering process by controlling the composition of the alloy steel. The present invention provides lower costs, and further provides an alloy steel having excellent strength and toughness as compared to a conventional alloy steel.

2. Description of the Related Art

Studies for suppressing the discharge of exhaust gas, such as carbon dioxide gas and the like, have been ongoing due to the recently highlighted global warming problems. The automobile industry has attempted to improve power output and fuel efficiency of an engine by making automobile steel parts more lightweight, and has further attempted to reduce costs by simplifying the manufacturing process.

A hot-forged product in the related art is manufactured by heating a bar of material in the original material state at high temperature, forging the bar, allowing the bar to cool by air, and then performing oil quenching after maintaining the temperature by reheating the bar at the AC3 temperature or more. While the material is provided with strength through such heat treatment, there is a problem in that ductility and toughness are reduced.

In order to increase toughness, residual stress in the material can be removed by performing tempering at the AC1 temperature or more for an extended period of time. However, this is problematic because the forging and heat treatment are expensive and time consuming because they include a plurality of processes and require maintaining a high temperature for an extended period of time.

Accordingly, a non-quenched and tempered steel is generally used in major automobile moving parts, such as a connecting rod, a crank shaft, and the like. A non-quenched and tempered steel means a material which allows the omission of thermal refining (quenching and tempering) on an alloy steel, and is instead subjected to slow cooling treatment after the steel is forged.

While the non-quenched and tempered steel is advantageous in cost and in that the entire manufacturing process is shortened due to the omission of thermal refining, mechanical properties, such as toughness, strength and the like, are deteriorated as compared to the alloy steel in the related art. As such, application of the non-quenched and tempered steel to parts requiring high strength or high toughness is problematic.

SUMMARY OF THE INVENTION

The present invention provides an alloy steel for hot forging and a heat treatment Method thereof, wherein heat treatment processes, such as reheating, tempering and the like, can be omitted, particularly by controlling the composition ratio of the alloy steel. As such, direct quenching can be applied to the present alloy steel to provide excellent strength and toughness as compared to the alloy steel in the related art.

According to one aspect, the present invention provides an alloy steel for hot forging, including iron as a base material (i.e. main component), and further including carbon (C), manganese (Mn), and molybdenum (Mo). According to a preferred embodiment, the present invention provides an alloy steel for hot forging, including iron as a base material, about 0.08 to 0.13% by weight of carbon (C), about 1.3 to 1.5% by weight of manganese (Mn), and about 0.1 to 0.3% by weight of molybdenum (Mo), based on a total weight of the alloy steel.

According to various embodiments of the present invention, the alloy steel further includes niobium (Nb), preferably about 0.01 to 0.05% by weight of niobium (Nb).

According to various embodiments of the present invention, the alloy steel further includes one or more of silicon (Si), chromium (Cr) and boron (B), preferably about 0.4 to 1% by weight of silicon (Si), about 0.8 to 1.2% by weight of chromium (Cr), and about 0.003% by weight or less of boron (B).

According to various embodiments of the present invention, the alloy steel further includes one or more of titanium (Ti) and aluminum (Al), preferably about 0.01 to 0.04% by weight of titanium (Ti) and about 0.01 to 0.04% by weight of aluminum (Al).

According to another aspect, the present invention provides a heat treatment method of an alloy steel for hot forging, including: hot forging the alloy steel having the aforementioned composition; allowing the hot-forged alloy steel to air cool in a temperature range from about 970° C. to 1,010° C.; and quenching the air-cooled alloy steel.

According to an exemplary embodiment of the present invention, the hot forging is performed in a temperature range from about 1,150° C. to 1,250° C.

According to an exemplary embodiment of the present invention, the quenching is performed by oil quenching using a quenching oil, and it is more preferred that a temperature of the quenching oil is about 55° C. or less.

The alloy steel having the aforementioned configuration according to the present invention has excellent toughness, strength and the like as compared to the non-quenched and tempered steel and the alloy steel for hot forging in the related art. As such, the alloy steel of the present invention may be applied to parts which are operated under rigorous conditions.

Further, according to the present invention, air cooling, reheating, and tempering may be omitted unlike the alloy steel for hot forging in the related art. As such, the present invention produces a highly efficient and environmentally friendly product that reduces processing time and cost, improves in productivity, and decreases the amount of carbon dioxide discharged.

In addition, the present alloy steel has a high strength that allows for downsizing of parts. As such, fuel efficiency is improved by providing more lightweight automobile parts fabricated from the alloy steel of the present invention. Other features and aspects of the present invention will be apparent from the following detailed description, drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the present invention will now be described in detail with reference to certain exemplary embodiments thereof illustrated the accompanying drawings which are given hereinbelow by way of illustration only, and thus are not limitative of the present invention, and wherein:

FIG. 1 is a graph illustrating a conventional heat treatment process of an alloy steel for hot forging.

FIG. 2 is a graph illustrating a heat treatment process of an alloy steel for hot forging according to an embodiment of the present invention.

It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various preferred features illustrative of the basic principles of the invention. The specific design features of the present invention as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in part by the particular intended application and use environment.

In the figures, reference numbers refer to the same or equivalent parts of the present invention throughout the several figures of the drawing.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Terms or words used in the present specification and claims should not be interpreted as being limited to typical or dictionary meanings, but should be interpreted as having meanings and concepts, which comply with the technical spirit of the present invention, based on the principle that an inventor can appropriately define the concept of the term to describe his/her own invention in the best manner. Therefore, configurations illustrated in the embodiments and the drawings described in the present specification are only the most preferred embodiment of the present invention and do not represent all of the technical spirit of the present invention, and thus it is to be understood that various modified examples, which may replace the configurations, are possible when filing the present application. It is understood that the term “vehicle” or “vehicular” or other similar term as used herein is inclusive of motor vehicles in general such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like, and includes hybrid vehicles, electric vehicles, plug-in hybrid electric vehicles, hydrogen-powered vehicles and other alternative fuel vehicles (e.g. fuels derived from resources other than petroleum). As referred to herein, a hybrid vehicle is a vehicle that has two or more sources of power, for example both gasoline-powered and electric-powered vehicles.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the Presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

Unless specifically stated or obvious from context, as used herein, the term “about” is understood as within a range of normal tolerance in the art, for example within 2 standard deviations of the mean. “About” can be understood as within 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, 0.1%, 0.05%, or 0.01% of the stated value. Unless otherwise clear from the context, all numerical values provided herein are modified by the term “about”.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

In an aspect, the present invention relates to an alloy steel for hot forging, which has better strength, toughness, and the like than the conventional alloy steel. According to the present invention, the alloy steel is provided with such properties even though only direct quenching is applied after hot forging.

Table 1 is a table which compares the composition of a quenched and tempered steel (SCM440) used in knuckles, axle beams, and the like among commercial vehicle body parts (a) in the related art and (b) with the composition of an alloy steel for hot forging according to an embodiment of the present invention.

TABLE 1 C Si Mn Cu Cr Mo Ni Nb Ti Al B The present about about about — about about — about about about about invention 0.08 to 0.40 to 1.30 to 0.80 to 0.10 to 0.01 to 0.01 to 0.01 to 0.003 or 0.13 1.00 1.50 1.20 0.30 0.05 0.04 0.04 less Related art 0.38 to 0.15 to 0.60 to 0.30 0.90 to 0.15 to 0.25 — — — — (%) 0.43 0.35 0.85 or less 1.20 0.30 or less

As illustrated in the table, the quenched and tempered steel applied to knuckles, axle beams, and the like among commercial vehicle body parts in the related art includes iron (Fe) as a base material, and 0.38 to 0.43% by weight of carbon (C), 0.15 to 0.35% by weight of silicon (Si), 0.60 to 0.85% by weight of manganese (Mn), 0.30% by weight or less of copper (Cu), 0.90 to 1.20% by weight of chromium (Cr), 0.15 to 0.30% by weight of molybdenum (Mo), and 0.25% by weight or less of nickel (Ni), based on the total weight of the quenched and tempered steel.

On the other hand, in the alloy steel for hot forging according to the present invention, manganese (Mn) and molybdenum (Mo) are added to a low-carbon base so as to have Bainite and Martensite structures when the alloy is formed, thereby solving the toughness-related problem of the quenched and tempered steel and further securing strength of the non-quenched and tempered steel.

Specifically, it is preferred that the alloy steel for hot forging according to the present invention includes iron as a base material, and about 0.08 to 0.13% by weight of carbon (C), about 1.3 to 1.5% by weight of manganese (Mn), and about 0.1 to 0.3% by weight of molybdenum (Mo), based on the total weight of the alloy steel. In particular, the iron is a main component that makes up the balance of the alloy steel composition.

(A) Carbon (C)

First, the carbon (C) is an element which increases the strength and hardness of a material, and is essential for a fine alloy element to precipitate a carbide. According to preferred embodiments, carbon (C) is included in an amount of about 0.08% by weight or more in consideration of improvement in the mechanical strength and in an amount of about 0.13% by weight or less in consideration of securing a Bainite structure and reducing toughness.

(B) Manganese (Mn)

The manganese (Mn) is an element which makes perlite fine and achieves solid-solution strengthening of ferrite. According to preferred embodiments, manganese (Mn) is included in an amount of about 1.3% by weight or more in order to improve the yield strength and in an amount of about 1.5% by weight or less in order to prevent cracks or modifications from being generated during the quenching.

(C) Molybdenum (Mo)

The molybdenum (Mo) is an element which improves hardenability and tempering embrittlement. According to preferred embodiments, molybdenum (Mo) is included in an amount of about 0.1% by weight or more in order to form a fine Bainite structure, and in an amount of about 0.3% by weight or less in consideration of satisfaction of the desired effects (e.g. hardenability and tempering embrittlement) and costs.

According to various embodiments, it is preferred that the alloy steel for hot forging further includes niobium (Nb), preferably about 0.01 to 0.05% by weight of niobium (Nb) in order to improve mechanical properties, such as strength and the like.

(D) Niobium (Nb)

The niobium (Nb) is an element which prevents crystal grains of steel from being coarsened and improves physical properties such as ductility, toughness, strength and the like by making crystal grains fine. According to preferred embodiments, the element is included in an amount of about 0.01% by weight or more for these effects (e.g. preventing coarse crystal grains of steel and improving physical properties) and in an amount of about 0.05% by weight or less in consideration of satisfaction of the aformentoned effects, and to maintain low material costs.

According to various embodiments, it is preferred that the alloy steel for hot forging according to the preset invention further includes one or more of silicon (Si), chromium (Cr), and boron (B), preferably about 0.4 to 1% by weight of silicon (Si), about 0.8 to 1.2% by weight of chromium (Cr), and about 0.003% by weight or less of boron (B) in order to improve hardenability, strength, and the like.

(E) Silicon (Si)

The silicon (Si) is an element which serves as an effective deoxidizer during steel-making and contributes to the improvement in yield strength and the like. According to preferred embodiments silicon (Si) is included in an amount of about 0.4% by weight or more in order to improve strength and in an amount of about 1% by weight or less to prevent deterioration in toughness and to maintain plastic workability.

(F) Chromium (Cr)

The chromium (Cr) is an element which increases the hardness of a low-carbon base and improves wear resistance, hardenability and the like. According to preferred embodiments, chromium (Cr) is included in an amount of about 0.8% by weight or more for these effects (e.g. hardness, wear resistance, hardenability) and in an amount of about 1.2% by weight or less in consideration of satisfaction degree of these effects, to prevent increase in costs of material due to high prices, and the like.

(G) Boron (B)

The boron (B) is an element which is arbitrarily added in order to improve hardenability. In particular, the hardenability is significantly increased even by adding a small amount thereof. According to preferred embodiments, boron (B) is included in an amount of about 0.003% by weight or less because the addition in a large amount causes brittleness.

According to various embodiments, it is preferred that the alloy steel for hot forging further includes one or more of titanium (Ti) and aluminum (Al), preferably about 0.01 to 0.04% by weight of titanium (Ti) and about 0.01 to 0.04% by weight of aluminum (Al) in order to further improve hardenability, strength, and the like.

(H) Titahium (Ti)

The titanium (Ti) is an element which improves hardenability, strength, and the like. According to preferred embodiments, titanium (Ti) is included in an amount of about 0.01% by weight or more for these effects (e.g. hardenability, strength) and in an amount of about 0.04% by weight or less in consideration of reduction in fatigue strength by potential formation of TiN compounds.

(I) Aluminum (Al)

The aluminum (Al) is an element which improves strength, and the like as crystal grains are made fine. According to preferred embodiments, aluminum (Al) is included in an amount of about 0.01% by weight or more in consideration of reduction in strength caused by coarsening of crystal grains and in an amount of about 0.04% by weight or less in order to prevent brittleness from being increased by precipitation of Al₂O₃ and the like which may be present.

In the alloy steel for hot forging having the aforementioned composition according to the present invention, the conventional tempering process can be omitted, and only a quenching process after hot forging can be carried out so as to provide an alloy steel that exhibits excellent physical properties as compared to the conventional alloy steel. In other words, directly after a hot forging process, quenching is carried out subsequent to an air cooling step. In another aspect, the present invention relates to a heat treatment process which is optimized in order to obtain optimal physical properties using an alloy steel having the aforementioned composition.

FIG. 1 is a graph illustrating a heat treatment process of an alloy steel for hot forging in the related art. As illustrated, the steel is hot-forged and then cooled, and is subjected to oil quenching after maintaining the temperature by reheating the alloy at the AC3 temperature or more, and then subjected to tempering in order to impart appropriate toughness.

FIG. 2 is a graph illustrating a heat treatment process of an alloy steel for hot forging according to the present invention. As illustrated, after the alloy steel is hot-forged in a temperature range from about 1,150° C. to 1,250° C., and preferably at about 1,200° C., the hot-forged alloy steel is allowed to air cool in a temperature range from about 970° C. to 1,010° C., and then is immediately quenched.

At this time, the quenching of the present invention is preferably performed by oil quenching. In particular, it is preferred that a quenching oil used for oil quenching is maintained at about 50 to about 60° C., and preferably 55° C. or less.

The air cooling temperature range from about 970° C. to 1,010° C. is a temperature range which is optimized in order to exhibit the best physical properties of the alloy steel for hot forging according to the present invention. Further, an additional process of air cooling may be included in the process after quenching, such as transportation and the like.

EXAMPLE 1

Hereinafter, the present invention will be described in more detail through Examples. These Examples are only for illustrating the present invention, and it will be obvious to those skilled in the art that the scope of the present invention is not interpreted to be limited by these Examples.

Among commercial vehicle body parts, knuckles, axle beams, and the like are mass produced by subjecting a conventional alloy steel (SCM440) for hot forging to thermal refining. Most of the parts commercially used have a weight of 30 to 60 kg for material introduced, and the size and weight thereof are considerable. Thus, it is important to obtain uniform heat treatment physical properties.

Meanwhile, the alloy steel for hot forging, which satisfies the composition according to the present invention, was cut off, (i.e. a piece of the alloy steel was cut for testing) heated up to 1,200° C., and then was subjected to hot forging. The forging was a hammer forging, and the alloy steel was subjected to a forging process at a rate of about 20 strokes. The forged product was then molded and moved by a conveyor belt, and when the temperature reached about 1,000° C., direct quenching was performed in a tank in which quenching oil was added.

In the aforementioned process, the time for heat treatment was reduced (about 5 hours) as compared to a hot forging process in the related art because the product was hot-forged, and then air cooled, and as a result, a process of reheating (typically about 2 hours) up to 850° C. and tempering (typically about 3 hours) was omitted.

TABLE 2 Tensile Hardness strength Yield strength Impact strength Reduction in (HRC) (MPa) (MPa) (J/cm²) Elongation (%) area (%) Example 1 38 to 40 1,200 1,030 140 to 160 15 60

Table 2 shows a result obtained by performing a test of physical properties with forged products completed after the alloy steel according to the present invention was subjected to heat treatment. As shown, the forged products satisfied all of the preferable standards of the alloy steel for hot forging in the related art, such as tensile strength of 980 MPa or more, an impact strength of 60 J/cm² or more, and an elongation of 12% or more.

In addition, a durability test was performed with hot-forged products of the present invention subjected to the process, and a more rigorous durability test (15 tons) in a vertical direction as compared to the existing test conditions (7.35 tons) was performed for discrimination of durability test results. As a result, there was an effect of improving strength by about 33% or more compared to the hot-forged product in the related art. Further, as a result of performing a test of physical properties by preparing a specimen in a state of the forged product of the present invention after the test, it was confirmed that tensile strength, yield strength, impact strength, and elongation were improved by approximately 17%, 14%, 60%, and 20%, respectively, as compared to the hot-forged product in the related art.

TABLE 3 Actual durability conditions Rigorous conditions Comparative Comparative Classification Example 1 Example 1 Example 1 Example 1 Primary # 1 Satisfactory Satisfactory 240,000 times 360,000 mock-up times (Mega # 2 Satisfactory Satisfactory 200,000 times 370,000 Truck) times # 3 Satisfactory Satisfactory 210,000 times 310,000 times

Table 3 is a table showing the result of the axle beam test of a commercial vehicle. Further, a durability test was performed under test conditions of an actual durability condition of 7.37 tons and a rigorous condition of 15 tons by preparing each of the three samples of the steel for hot forging, which satisfied steel for hot forging in the related art and the composition of the present invention. The actual durability condition goal is 300,000 times or more, the durability life span is 700,000 km or more, and the rigorous conditions are a simple comparison.

As a result, all of the actual durability conditions were satisfied, and the alloy steel for hot forging according to the present invention showed excellent results under rigorous conditions as compared to the alloy steel in the related art.

As described above, the present invention has been described in relation to specific embodiments of the present invention, but this is only illustration and the present invention is not limited thereto. Embodiments described may be changed or modified by those skilled in the art to which the present invention pertains without departing from the scope of the present invention, and various alterations and modifications are possible within the technical spirit of the present invention and the equivalent scope of the claims which will be described below. 

What is claimed is:
 1. An alloy steel for hot forging comprising: iron as a base material, and about 0.08 to 0.13% by weight of carbon (C), about 1.3 to 1.5% by weight of manganese (Mn), and about 0.1 to 0.3% by weight of molybdenum (Mo), based on a total weight of the alloy steel.
 2. The alloy steel for hot forging of claim 1, further comprising: about 0.01 to 0.05% by weight of niobium (Nb).
 3. The alloy steel for hot forging of claim 1, further comprising: about 0.4 to 1% by weight of silicon (Si), about 0.8 to 1.2% by weight of chromium (Cr), and about 0.003% by weight or less of boron (B).
 4. The alloy steel for hot forging of claim 1, further comprising: about 0.01 to 0.04% by weight of titanium (Ti) and about 0.01 to 0.04% by weight of aluminum (Al).
 5. A heat treatment method of an alloy steel for hot forging, the method comprising: hot forging the alloy steel of claim 1; allowing the hot-forged alloy steel to air cool in a temperature range from about 970° C. to 1,010° C.; and quenching the alloy steel air-cooled in the temperature range.
 6. The method of claim 5, wherein the hot forging is performed in a temperature range from about 1,150° C. to 1,250° C.
 7. The method of claim 5, wherein the quenching is performed by oil quenching using a quenching oil.
 8. The method of claim 7, wherein a temperature of the quenching oil is about 55° C. or less.
 9. The method of claim 5, wherein allowing the hot-forged alloy steel to air cool and quenching are carried out directly after the hot forging. 